İ YTE - Department YTE Department of Electrical Electrical and Electronics Electronics Engineering Engineering
EE315 - Electronics Laboratory
Experiment 4 BJT Small Signal Analysis Preliminary Work 1.a) 1.a) Calculate Av, Ai, Rin, and Rout on the circuit given in Figure 1. Assume that CCi and CCo behave as open circuit for DC signals and as short circuit for AC signals. source, vS, for the transistor to remain in forward1.b) Determine limits of the signal source, active region. VCC=15V
CCi RS μF 4.7 100Ω
R1 110kΩ
Q1: BC237 or BC238 β=200 VBE(on) =0.7V
RC 3.6kΩ
Q1 R2 27kΩ
vS
vo
CCo 4.7μF
RE 1.2kΩ
Rin
RL 12kΩ Rout
Figure 1. Common-emitter amplifier.
2.a) 2.a) Calculate Av, Ai, Rin, and Rout on the circuit given in Figure 2. Assume that the coupling capacitors, CCi and CCo, behave as open circuit for DC signals and as short circuit for AC signals. source, vS, for the transistor to remain in forward2.b) Determine limits of the signal source, active region. region. (Hint: AC current on RL will be added onto the quiescent emitter current.) VCC=15V CCi RS 100Ω 4.7μF
RB 220kΩ
Q1: BC237 or BC238 β=200 VBE(on)=0.7V
Q1
vS
CCo 4.7μF
RE 1.2kΩ Rin
vo RL 100Ω
Rout
Figure 2. Common-collector amplifier (emitter follower).
EE315 - Electronics Laboratory - Experiment 4
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İ YTE - Department of Electrical and Electronics Engineering
Procedure 1. Build the circuit given in Figure 1. Use the function generator to obtain 1 Vp-p, 1 KHz as vS. Note the polarity of the capacitors on the circuit diagram. 1.a) Measure the voltage gain, Av. Use the oscilloscope in X-Y mode, probing vS on Ch-1 (X) and vO on Ch-2 (Y). VSp-p =
VOp-p =
Av =
1.b) Determine the current gain, Ai, by measuring the peak-to-peak AC voltages across RS and RL. VRSp-p =
VRLp-p =
ISp-p =
ILp-p =
Ai =
1.c) Disconnect RL and measure the input resistance, Rin. Use the oscilloscope in X-Y mode, probing iS (voltage across RS) on Ch-1 (X) and vS on Ch-2 (Y). Note that, the two oscilloscope channels must share a single, common node for proper ground connection. VSp-p =
VRSp-p =
ISp-p =
Rin =
1.d) Measure peak-to-peak output voltage while RL is disconnected. Connect a 4.7 kΩ variable resistor in place of RL, and adjust the variable resistor to obtain half the output voltage measured without any load. Measure the value of variable resistor that will give the Rout of the amplifier. VOp-p(NoLoad) =
Rout =
1.e) Find the maximum peak vS amplitudes before the transistor goes into cut-off or saturation. Compare the results with your calculations in the preliminary work. VSpeak(cut-off) =
VSpeak(sat) =
1.f) Connect a 4.7 μF capacitor in parallel to RE. Make sure that the polarity of the capacitor matches the DC voltage polarity on RE. Measure the voltage gain, Av, again as described in step 1.a. VSp-p =
VOp-p =
Av =
1.g) Draw the small signal model of the common-emitter amplifier with the capacitor in parallel to RE, and calculate the voltage gain, Av, based on your model.
EE315 - Electronics Laboratory - Experiment 4
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İ YTE - Department of Electrical and Electronics Engineering
2. Build the emitter-follower circuit given in Figure 2. Verify that the function generator output is still set to 1 Vp-p, 1 KHz, and repeat the steps 1.a, 1.b, and 1.c. 2.a) Measure the voltage gain, Av. VSp-p =
VOp-p =
Av =
2.b) Determine the current gain, Ai. VRSp-p =
VRLp-p =
ISp-p =
ILp-p =
Ai =
2.c) Disconnect RL and measure the input resistance, Rin. VSp-p =
VRSp-p =
ISp-p =
Rin =
2.d) Increase vS amplitude to 2.0 Vp-p and observe vO on the oscilloscope. How do you explain the distortion in the vO waveform?
2.e) Find a way to restore the vO waveform while keeping vS amplitude at 2.0 Vp-p, and RL = 100 Ω. Explain how you can restore the vO waveform.
2.f) Set the function generator output back to 1 Vp-p, and measure the output resistance, Rout. You should use a smaller variable resistor such as 100 Ω to obtain a better precision with emitter-follower. Make sure that the output waveform is not distorted while you measure Rout. VOp-p(NoLoad) =
Rout =
EE315 - Electronics Laboratory - Experiment 4
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